Voltage Drop Calculator
Wire Run Voltage Loss Estimation
Voltage Drop
Ready for Calculation
Select your wire size and enter circuit details to estimate voltage drop.
Understanding Voltage Drop
Every conductor has resistance, so some voltage is inevitably lost as current travels along a wire. Over long runs or with undersized conductors, this loss can become significant enough to dim lights, overheat motors, or trip sensitive electronics.
Distance & Gauge
Voltage drop increases with distance and current, and decreases as wire gauge gets thicker (a lower AWG number). Doubling the distance roughly doubles the drop for the same wire and load.
Copper vs. Aluminum
Copper is a better conductor than aluminum, so an aluminum run typically needs a thicker gauge than copper to achieve the same voltage drop over the same distance.
The 3% / 5% Guideline
The National Electrical Code (NEC) recommends keeping voltage drop under 3% for branch circuits and under 5% for the combined feeder and branch circuit. This isn't a strict legal maximum in most cases, but exceeding it can cause noticeably dimmer lights, hotter-running motors, and reduced equipment lifespan.
Long Runs Need Bigger Wire
For long outdoor runs (well pumps, detached garages, sub-panels), upsizing the wire gauge is often the only practical way to stay within guidelines.
Key Takeaways
- Round-Trip Distance: This calculator uses one-way distance and automatically accounts for the return path in single-phase calculations.
- Estimate Only: Results use standard resistivity constants and don't account for temperature, conduit fill, or exact conductor stranding — consult an electrician for critical installations.
- Code Compliance: Always verify final wire sizing against your local electrical code, not just the voltage-drop guideline alone.